1. Field of the Invention
This invention relates to methods and compositions for the prevention of dental caries.
2. Description of Related Art
The ability of a given bacterium to colonize a host is determined by a number of factors such as the bacterium's metabolic needs, and the interactions of the bacterium with the pre-existing bacterial flora. Bacterial interactions can be generally classified as either "positive" or "negative." In positive interactions, an "effector" bacterial strain alters the microenvironment to promote colonization of a second, "target" organism. In "negative" interactions, the effector strain alters the microenvironment in a manner that decreases or completely prevents target bacterium colonization. Negative interactions between competing bacteria during host colonization are commonly termed bacterial interference.
Therapeutic regimens which take advantage of bacterial interference to replace a pathogenic bacterial strain with a non-pathogenic, effector strain are termed replacement therapies. Successful replacement therapy requires an effector strain that: 1) is non-pathogenic, 2) alters the microenvironment so as to prevent colonization or outgrowth of pathogenic organisms, 3) persistently colonizes the host at risk to prevent reinfection by the target pathogenic organism, and aggressively displace the pathogenic organism from the tissues at risk where the pathogen is part of the host's indigenous flora.
It is well known that dental caries are caused by bacteria that colonize the oral cavity. The principle dental caries-causing bacterial pathogen in humans is the bacterium Streptococcus mutans. The characteristic features of S. mutans implicated in cariogenesis include its ability to produce lactic acid, as well as its ability to accumulate on tooth enamel (i.e., plaque formation) (Gibbons et al., 1969, J. Bacteriol., 98.:341-346; Makinen et al., 1972, Int. Dent. J., 22:362-386; and Jordan, 1965, Ann. N.Y. Acad. Sci., 131:905-912). Application of the principles of replacement therapy would require the isolation of a non-cariogenic effector strain of S. mutans, e.g., a S. mutans strain deficient in lactic acid synthesis. The progress in the application of replacement therapy to dental caries has been recently reviewed (Hillman et al., 1989, In: New Biotechnologies in Oral ReSearch, H. M. Myers, ed., S. Karger, Basel, Switzerland, pgs. 1-17).
Generation of a lactic acid-deficient S. mutans, and thus a strain suitable for replacement therapy for dental caries, has met with considerable difficulty. Defects in lactic acid synthesis are lethal to S. mutans. Abhyanakar et al. (1985, J. Dent. Res., 64:1267-1271) generated a lactate dehydrogenase(LDH)-deficient S. mutans mutant by chemical mutagenesis using an atypical S. mutans strain that contained a preexisting (spontaneous) mutation affecting pyruvate metabolism. However, the chemically-induced and the spontaneously-generated mutations in this strain remain uncharacterized. Moreover, chemically-induced, uncharacterized mutations can revert to wild-type, and thus the pathogenic, cariogenic phenotype. Subsequent attempts to produce LDH-deficiencies in other S. mutans strains have failed (Hillman et al., 1994, Infect. Immun., 62:60-64; Chen et al., 1994, J. Bacteriol., 176:1542-1545).
There is a clear need in the field for a stable, lactic acid-deficient, non-cariogenic strain of S. mutans that is suitable for use in a replacement therapy in the prevention and/or treatment of dental caries.